Surface Plasmon Spectroscopy of Nanosized Metal

Surface Plasmon Spectroscopy of Nanosized Metal

(Parte 1 de 5)

Surface Plasmon Spectroscopy of Nanosized Metal Particles

Paul Mulvaney

Advanced Mineral Products Research Centre, School of Chemistry, University of Melbourne, Parkville, Victoria, 3052, Australia

Received April 4, 1995. In Final Form: July 7, 1995X

The use of optical measurements to monitor electrochemical changes on the surface of nanosized metal particles is discussed within the Drude model. The absorption spectrum of a metal sol in water is shown to be strongly affected by cathodic or anodic polarization, chemisorption, metal adatom deposition, and alloying. Anion adsorption leads to strong damping of the free electron absorption. Cathodic polarization leadstoaniondesorption. Underpotentialdeposition(upd)ofelectropositivemetallayersresultsindramatic blue-shifts of the surface plasmon band of the substrate. The deposition of just 0.1 monolayer can be readily detected by eye. In some cases alloying occurs spontaneously during upd. Alloy formation can be ascertained from the optical absorption spectrum in the case of gold deposition onto silver sols. The underpotential deposition of silver adatoms onto palladium leads to the formation of a homogeneous silver shell, but the mean free path is less than predicted, due to lattice strain in the shell.

Introduction

Interestintheopticalpropertiesofcolloidalmetalsdates backtoRomantimes. Nanosizedgoldparticleswereoften used as colorants in glasses, and quite complex optical effects were created using metal particles.1 In the seventeenth century, “Purple of Cassius”, a colloid of heterocoagulated tin dioxide and gold particles, became apopularcolorantinglasses.2 Theseearlymanifestations oftheunusualcolorsdisplayedbymetalparticlesprompted Faraday’s investigations into the colors of colloidal gold in the middle of the last century. Today his studies are generally considered to mark the foundations of modern colloid science.3 The formation of color centers and small colloidal metal particles in ionic matrices and glasses has remained an area of very active research,4-6 driven, in part, by the technical importance of the photographic process.7 However,colloidchemistshavetendedtoneglect the study of metal particles in aqueous solution because oftheircomplicateddoublelayerstructure,whichismore amenable to direct electrochemical investigation. The morerecentdiscoverythatthesurfaceplasmonabsorption band can also provide information on the development of the band structure in metals8-1 has led to a plethora of studies on the size dependent optical properties of metal particles, particularly those of silver and gold,12-17 while advances in molecular beam techniques now enable spectroscopic analysis of metal clusters to be carried out in vacuum.18,19

Although many of the optical effects associated with nanosized metal particles are now reasonably well understood,therearelargediscrepanciesbetweentheoptical properties of metal sols prepared in water, particularly those of silver, and sols prepared in other matrices.6,20-27 In a recent review Kreibig noted that while much work has been done to isolate matrix effects and to determine the roles of defects, grain boundaries, crystallinity, and polydispersity on the optical properties of sols, little is knownaboutthewayspecificsurfacechemicalinteractions may influence the absorption of light by small metal particles.28 These differences are attributed to unique doublelayereffectspresentatthemetal-waterinterface. This review focuses on some of these surface chemical effects, and attempts to show how changes to the surface plasmon absorption band of aqueous metal colloids can berelatedtoelectrochemicalprocessesoccurringatmetal particle surfaces. Simple models are proposed to explain some of these chemical changes within the Drude framework for surface plasmon absorption.

1. Light Absorption by Colloids

In the presence of a dilute colloidal solution containing N particles per unit volume, the measured attenuation of light of intensity Io, over a pathlength d cm is given byX Abstract published in Advance ACS Abstracts, December 15, 1995.

(1) See, for example: Savage, G. Glass and Glassware; Octopus

Books: London,1975.OneofthemostfamousexamplesistheLycurgus Cupwhichisrubyredintransmittedlightbutappearsgreeninreflected light. The color is due to colloidal gold. It was manufactured in the 4th century AD. (2) See: Thiessen, P. A. Kolloid Z. 1942, 101, 241, for micrographs of this composite. (3) Faraday, M. Philos. Trans. R. Soc. 1857, 147, 145. (4) Siedentopf, H. Z. Phys. 1905, 6, 855. (5) Mott, N. F.; Gurney, R. W. Electronic Processes in Ionic Crystals;

Oxford University Press: Oxford, 1948. (6) Hughes, A. E.; Jain, S. C. Adv Phys. 1979, 28, 717. (7) The Theory of the Photographic Process, 4th ed.; James, T. H.,

Ed.; MacMillan Press: New York, 1977. (8) Scott, A. B.; Smith, W. A.; Thompson, M. A. J. Phys. Chem. 1953, 57, 757. (9) Doremus, R. H. J. Chem. Phys. 1965, 42, 414. (10) Doyle, W. T. Phys. Rev. 1958, 1, 1067. (1) Romer, H.; von Fragstein, C. Z. Phys. 1961, 163, 27. (12) Perenboom, J. A. A.; Wyder, P.; Meier, F. Phys. Rep. 1981, 78, 173. (13) Papavassiliou, G. C. Prog. Solid State Chem. 1980, 12, 185. (14) Kreibig, U. J. Phys. F: Met. Phys. 1974, 4, 9.

(15) von Fragstein, C.; Schoenes, F. J. Z. Phys. 1967, 198, 477. (16) Kreibig, U. Z. Phys. B: Condens. Matter Quanta 1978, 31, 39;

J. Phys. (Paris) 1977, 38,C 2-97. (17) Yanase, A.; Komiyama, H. Surf. Sci. 1991, 248, 1, 20. (18) Fallgren, H.; Martin T. P.; Chem. Phys. Lett. 1990, 168, 233. (19) (a)Tiggesbaumker,J.;Koller,L.;Meiwes-Broer,K.-H.;Liebsch,

A. Phys. Rev. A 1993, 48, R1749. (b) Huffman, D. R. Adv. Phys. 1977, 26, 129. (20) Frens, G.; Overbeek, J. Th. G. Kolloid Z. Z. Polym. 1969, 233, 922. (21) Berry, C. R.; Skillman, D. C. J. Appl. Phys. 1971, 42, 2818. (2) Miller, W. J.; Herz, A. H. In Colloid and Interface Science;

Academic Press: New York, 1976; Vol. 4. (23) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sanders, J. V. J.

Colloid Interface Sci. 1983, 93, 545. (24) Henglein, A. J. Phys. Chem. 1979, 83, 2209. (25) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (26) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem.

Soc., Faraday Trans. 2 1979, 75, 790. (27) Linnert,T.;Mulvaney,P.;Henglein,A.J.Phys.Chem.1993,97, 679. (28) Kreibig, U.; Genzel, U. Surf. Sci. 1985, 156, 678.

0743-7463/96/2412-0788$12.0/0 © 1996 American Chemical Society

whereCextistheextinctioncrosssectionofasingleparticle. For spherical particles with a frequency dependent

where k ) 2ð x m/ì and an and bn are the scattering coefficients, which are functions of the radius R and the wavelength ì in terms of Ricatti-Bessel functions. The extinction cross section of a particle is often normalized to give the extinction cross section per unit area:

Conventionally, chemists measure the extinction coefficientofasolutioninunitsofM-1cm-1,wherethecolloid concentrationisthemolarmetalatomconcentration.This

quantity is related to Qext by where Vm (cm3 mol-1) is the molar volume of the metal. For very small particles where kR , 1, only the first, electric dipole term in eq 2 is significant, and

Thisequationcanbealsoobtainedbypurelyelectrostatic arguments, and a clear derivation is given by Genzel and Martin.34 In many cases to be described here, it will be necessary to consider the perturbation introduced by a thinsurfacelayer. Theextinctioncrosssectionofasmall, concentric sphere is given by32

where core is the complex dielectric function of the core material, shell is that of the shell, m is the real dielectric function of the surrounding medium, g is the volume fractionoftheshelllayer,andRistheradiusofthecoated particle. Wheng)0,eq6reducestoeq5foranuncoated sphere, and for g ) 1, eq 6 yields the extinction cross section for a sphere of the shell material.

In the case of many metals, the region of absorption up tothebulkplasmafrequency(intheUV)isdominatedby the free electron behavior, and the dielectric response is well described by the simple Drude model. According to thistheory,35therealandimaginarypartsofthedielectric function may be written where ∞ is the high frequency dielectric constant due to interband and core transitions and öp is the bulk plasma frequency in terms of N, the concentration of free electrons in the metal, and m, the effective mass of the electron. öd is the relaxation or damping frequency, which is related to the mean free path of the conduction electrons, Rbulk, and the velocity of electrons at the Fermi energy, vf,b y

When the particle radius, R, is smaller than the mean free path in the bulk metal, conduction electrons are additionally scattered by the surface, and the mean free path, Reff, becomes size dependent with

Equation 1 has been experimentally verified by the extensiveworkofKreibigforbothsilverandgoldparticles right down to a size of 2 nm.14,16,28 The advantage of the Drude model is that it allows changes in the absorption spectrumtobeinterpreteddirectlyintermsofthematerial properties of the metal. The origin of the strong color changes displayed by small particles lies in the denominatorofeq5,whichpredictstheexistenceofanabsorption peak when

From eq 7 it can be seen that over the whole frequency regime below the bulk plasma frequency of a metal, ′ is negativewhichisduetothefactthattheelectronsoscillate outofphasewiththeelectricfieldvectorofthelightwave. This is why metal particles display absorption spectra which are strong functions of the size parameter, kR.I n a small metal particle the dipole created by the electric fieldofthelightwavesetsupasurfacepolarizationcharge, which effectively acts as a restoring force for the “free electrons”. The net result is that, when condition 12 is fulfilled,thelongwavelengthabsorptionbythebulkmetal is condensed into a single, surface plasmon band. In the case of semiconductor crystallites, the free electron concentration is orders of magnitude smaller, even in degenerately doped materials (i.e., öp is smaller), and as aresultsurfaceplasmonabsorptionoccursintheIR,rather than in the visible part of the spectrum. Semiconductor crystallites therefore do not change color significantly when the particle size is decreased below the wavelength ofvisiblelight,althoughtheIRspectrummaybeaffected. It should be noted that the strong color changes observed when semiconductor crystallites are in the quantum size regime (R < 50 Å), are due to the changing electronic band structure of the crystal, which causes the dielectric function of the material itself to change.

In Figure 1, a “typical” surface plasmon band is shown calculatedusingeq5withparameterstypicalofsilverfor several values of the damping parameter öd. The most importantparameteraffectingödistheparticlesize.From eqs 10 and 1 it can be seen that decreases in the particle sizeleadtoanincreaseinöd,causingthebandtobroaden and the maximum intensity to decrease. The position of the peak is virtually unaffected by small changes to öd

(29) Toon, O. B.; Ackerman, T. P. Appl. Opt. 1981, 20, 3657. (30) van der Hulst, H. C. Light Scattering by Small Particles; John

Wiley and Sons: New York, 1957. (31) Kurtz, V.; Salib, S. J. Imaging Sci. Technol. 1993, 37, 43. (32) Bohren,C.F.;Huffman,D.R.AbsorptionandScatteringofLight by Small Particles; Wiley: New York, 1983. (3) Kerker, M. The Scattering of Light and Other Electromagnetic

Radiation; Academic Press: New York, 1969. (34) Genzel, L.; Martin, T. P. Phys. Status Solidi B 1972, 51, 91. (35) Kittel, C. Introduction to Solid State Physics, 2nd ed.; Wiley: New York, 1956.

Optical Properties of Metal Particles Langmuir, Vol. 12, No. 3, 1996 789

butforlargedampingaslowshifttolowerenergiesoccurs. Inaninertmatrix,theonlycauseofpeakshiftsisachange in the dielectric properties of the metal particles themselves, due to this surface scattering or for exceedingly small particle sizes (<1-2 nm), to quantization of the energy levels within the conduction band. In the case of silver particles, quantization results in a blue-shift of the plasmon band and a break-up into discrete excitation bands.36-41 Nevertheless, in water, the experimentally measuredsurfaceplasmonabsorptionbandsofsilversols varyenormouslyinposition(rangingfrom375to405nm) and the absorption coefficients vary by factors of 3 or 420-27,42-45 These discrepancies cannot be explained on the basis of eq 2 and provided the motivation for much of the work to be described below. We will consider experimental spectroscopic data illustrating the effects of anion adsorption, electronic charging, and underpotentialmetaldeposition,andtrytointerpretthespectral featuresintermsofeqs5-8. Becausemetalparticlesare studiedinavarietyofmatrices,itisworthreviewingfirst how the surface plasmon absorption is affected by the solvent refractive index.

2. The Effect of the Solvent Refractive Index

ForsimplemetalsobeyingtheDrudemodel,theposition of the plasmon absorption peak does depend on the refractive index of the surrounding medium. Using eqs 5and6,wefind,forsmall ′′,thatthebandpositionshould obey46 length. From a plot of the observed band position vs 2 m, both the high-frequency dielectric constant and the bulk plasmafrequency(orwavelength)canbeextracted. Plots ofì2vs2 mareshownforsilverandleadcolloidsinFigure 2. From the data for lead we find that the bulk plasma energy is 1.3 eV and ∞ ) 1.1, in good agreement with electron loss spectroscopy data of Ashton and Green and that of Girault et al.47,48 In the case of silver, the band position in water is variable, but interpolating the values from the salt matrices, we predict that the “true” position ofthesilversurfaceplasmonbandinwateris382(1nm. The high frequency dielectric constant is estimated to be 5.9, not far from other estimates of 4.7-5.3. Adherence to eq 13 demonstrates that the absorption of light by the particles over the spectral region is due to the absorption byconductionelectrons,ratherthaninterbandtransitions. It also suggests that the position of the surface plasmon bandmaybeusedtomeasurethelocaldielectricconstant in microheterogeneous systems. Papavassiliou has used a similar procedure to analyze the optical properties of particulate metal films.49 He found that the color of the film was altered when it was immersed into solvents of differingrefractiveindex. Morerecently,Wilcoxonetal.50 and Esumi et al.51,52 have prepared metal sols in nonaqueousmedia,andobservedspectralshiftsinqualitative accord with eq 13. This opticalapproachto the measurement of the dielectric function of the environment is analogous to the use of solvatochromic molecules such as

ET30.53 Clearly, since the particles in these experiments are prepared in different media, and may have variable shapes,sizes,anddefectstructures,goodadherencetoeq 13cannotalwaysbeassumed. Ideally,thesameparticles

(36) Mulvaney, P.; Henglein, A. Chem. Phys. Lett. 1990, 168, 391. (37) Mulvaney, P.; Henglein, A. J. Phys. Chem. 1990, 94, 4182. (38) Henglein, A.; Mulvaney, P.; Linnert, T. Ber. Bunsen-Ges. Phys.

Chem. 1990, 94, 1449. (39) Mostafavi, M.; Marignier, J. L.; Amblard, J.; Belloni, J. Radiat.

(Parte 1 de 5)

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